2 (18.75 ounce) cans tomato bisque (such as Campbell's®)
2 tablespoons butter, softened
4 slices sourdough bread
4 slices aged Cheddar cheese
Pour 1 can tomato bisque into a microwave-safe bowl. Cover and cook in the microwave until hot, 2 1/2 to 3 minutes. Repeat with the remaining bisque.
Butter 1 side of each slice of bread. Place 2 slices of Cheddar cheese on the unbuttered sides of 2 slices. Top with the other 2 slices, buttered-side up, to make the sandwiches.
Heat a large skillet over medium-high heat. Add sandwiches. Heat until toasted and golden on 1 side, 2 to 3 minutes. Flip and toast the other side, about 2 minutes more. Cut off crusts, if desired. Slice sandwiches into 1 1/2-inch squares, 18 squares in total. Thread a toothpick through each square.
Pour heated soup into heat-proof shot glasses and top with the grilled cheese squares.